我在另一个问题中讨论了这个问题，但它偏离了原来的问题，所以我认为最好再提出一个问题。
下面是我的代码，所有内容都正确插入到 c_id 接受的子类别表中，每个条目都作为 0 插入到我的数据库中，这是代码。
<?php
//create_cat.php
include '../includes/connect.php';
include '../header.php';
echo '<h2>Create a Sub category</h2>';
if($_SESSION['signed_in'] == false | $_SESSION['user_level'] != 1 )
{
//the user is not an admin
echo 'Sorry, you do not have sufficient rights to access this page.';
}
else
{
//the user has admin rights
if($_SERVER['REQUEST_METHOD'] != 'POST')
{
//the form hasn't been posted yet, display it
echo '<form method="post" action="">
Category name: ';
$sql = "SELECT cat_id, cat_name, cat_description FROM categories";
$result = mysql_query($sql);
echo '<select name="topic_cat">';
while($row = mysql_fetch_assoc($result))
{
echo '<option value="' . $row['cat_id'] . '">' . $row['cat_name'] . '</option>';
}
echo '</select><br />';
echo 'Sub category name: <input type="text" name="sub_cat_name" /><br />
Sub category description:<br /> <textarea name="sub_desc" /></textarea><br /><br />
<input type="submit" value="Add Sub Category" />
</form>';
}
else
{
//the form has been posted, so save it
$sql = "INSERT INTO subcategories(c_id, sub_cat_name, sub_desc)
VALUES('" . $_POST['categories.cat_id'] . "', '" . $_POST['sub_cat_name'] . "', '" . $_POST['sub_desc'] . "')";
$result = mysql_query($sql) or die (mysql_error());
echo 'The sub category <b>' . $row['sub_cat_name'] . '</b> has been added under the main category <b>' . $row['cat_name'] . '</b>';
if(!$result)
{
//something went wrong, display the error
echo 'Error' . mysql_error();
}
}
}
; ?>